Gain-clamped semiconductor optical amplifier

Abstract

A gain-clamped semiconductor optical amplifier according to the present invention has a pair of DBR areas 2, 3 disposed in sandwiching relation to gain area 1 for amplifying guided light. A portion of a waveguide of gain area 1 comprises MMI waveguide 11.

Description

TECHNICAL FIELD[0001]The present invention relates to a semiconductor optical amplifier, and more particularly to a gain-clamped semiconductor optical amplifier.BACKGROUND ART[0002]Semiconductor optical amplifiers (SOAs) are promising as small-size, low-cost optical amplifiers to replace erbium-doped fiber amplifiers (EDFA) that are typically used in conventional optical communication systems.[0003]One of the goals to be achieved with respect to the performance of SOAs is to increase the saturated output. Generally, the maximum optical output of amplified light generated by SOAs is lower than that of EDFAs. At present, SOAs, which are considered to be more advantageous than EDFAs for their smaller sizes and lower costs, have not been available as a substitute for EDFAs. For SOAs to replace EDFAs in large-capacity wavelength-multiplex optical communication systems, the SOAs need to produce a saturated output of at least 20 dBm (100 mW). In efforts to increase the saturated output, ca...

AI Technical Summary

Benefits of technology

[0042]To achieve the above object, a gain-clamped semiconductor optical amplifier according to the present invention has a pair of reflectors disposed in sandwiching relation to a gain area for amplifying guided light, wherein a portion of a waveguide of the gain area comprises a multimode interference optical waveguide. With this arrangement, laser oscillation is caused by the reflectors to clamp a gain in the gain area to an oscillation threshold gain. Therefore, a degradation of a signal waveform due to a gain saturation and interchannel crosstalk upon amplification of wavelength-multiplexed signal light are suppressed. Since the portion of the waveguide of the gain area comprises a multimode interference optical waveguide, the area of the gain area (active layer) is increased, resulting in a corresponding reduction in the density of input light applied to the gain area (active layer) for an increased saturated output.

[0046]The gain-clamped semiconductor optical amplifier may further include a photodetector for detecting the clamped light emitted from the third or fourth port. With this arrangement, if the gain-clamped semiconductor optical amplifier is applied to a repeater amplifier, then the gain-clamped semiconductor optical amplifier operates as described above, and can also operate to produce a constant output by controlling the amount of a current injected into or a voltage applied to the gain adjusting waveguide in order to make the output of the photodetector constant. If the gain-clamped semiconductor optical amplifier is applied to a receiver in an optical communication system, then since the amplified clamped light is detected instead of the signal light, an error rate of signal conversion can be lowered.

[0047]The gain-clamped semiconductor optical amplifier may further include a photodetector for detecting the light emitted from the second port. With this arrangement, the gain-clamped semiconductor optical amplifier operates as described above, and can also lower an error rate of signal conversion.

Problems solved by technology

1) It is difficult to achieve both a high saturated output and polarization independence.

2) The drive current density is increased.

3) The gain saturation characteristics with respect to a signal light are deteriorated.

If the thickness of the active layer is reduced, a crystal strain required for polarization independence will be increased, resulting in crystal growth difficulty.

Therefore, it is difficult to achieve polarization-independent gain characteristics.

However, using such a high current density as a drive condition is not practical in view of deteriorated gain characteristics due to heating and device reliability.

With respect to the factor 3), the saturated output performance on dynamic characteristics for amplifying a modulated signal is not yet sufficient.

This is a problem caused by the definition of a saturated output which serves as an index for estimating the maximum optical output of an SOA.

SOAs, however, cause a signal waveform deterioration when driven in an optical output intensity range with a gain reduction of 3 dB.

Such a signal waveform deterioration is due to the essential nature of SOAs that they are susceptible to a gain saturation relaxation process because the carrier relaxation time in the active layer and the bit rate of the signal light are of about the same order of time.

As described above, it can bee seen that it is highly difficult to achieve a maximum optical output in excess of 20 dBm with the active layer according to the process of optimizing active layer while maintaining the single transverse mode.

In the gain saturation range, since the number of carriers consumed for amplifying the input light increases, the laser-oscillated state cannot be maintained, shutting off the gain clamping operation.

As described above, it is highly difficult to achieve a saturated output of at least 20 dBm mainly through the optimization of the active layer for the following reasons: 1) It is difficult to achieve both a high saturated output and polarization independence.

First, the problems of the process a) of improving the difficulties 1), 2) by increasing the effective waveguide width while maintaining a single transverse mode output will be described below.

It can be seen that because of the drive current, the present process is not sufficient for realizing an SOA which can be used as a substitute for an EDFA.

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